8.3.2 Application M13809-3 v8Two step residual overvoltage protection ROV2PTOV (59N) is primarily used in highimpedance grounded distribution networks, mainly as a backup for the primary groundfault protection of the feeders and the transformer. To increase the security for differentground fault related functions, the residual overvoltage signal can be used as a releasesignal. The residual voltage can be measured either at the transformer neutral or from avoltage transformer open delta connection. The residual voltage can also be calculatedinternally, based on the measurement of the three phase-to-ground voltages.In high impedance grounded systems the residual voltage will increase in case of anyfault connected to ground. Depending on the type of fault and fault resistance theresidual voltage will reach different values. The highest residual voltage, equal to threetimes the phase-to-ground voltage, is achieved for a single phase-to-ground fault. Theresidual voltage increases approximately to the same level in the whole system anddoes not provide any guidance in finding the faulted component. Therefore,ROV2PTOV (59N) is often used as a backup protection or as a release signal for thefeeder ground fault protection.8.3.3 Setting guidelines M13853-3 v8All the voltage conditions in the system where ROV2PTOV (59N) performs itsfunctions should be considered. The same also applies to the associated equipment, itsvoltage withstand capability and time characteristic.All voltage-related settings are made as a percentage of a settable base voltage, whichshall be set to the primary nominal voltage (phase-phase) level of the power system orthe high-voltage equipment under consideration.The time delay for ROV2PTOV (59N) is seldom critical, since residual voltage isrelated to ground faults in a high-impedance grounded system, and enough time mustnormally be given for the primary protection to clear the fault. In some more specificsituations, where the residual overvoltage protection is used to protect some specificequipment, the time delay is shorter.Some applications and related setting guidelines for the residual voltage level are givenbelow.8.3.3.1 Equipment protection, such as for motors, generators, reactors andtransformersEquipment protection for transformers M13853-9 v8High residual voltage indicates ground fault in the system, perhaps in the component towhich two step residual overvoltage protection (ROV2PTOV, 59N) is connected. Forselectivity reasons to the primary protection for the faulted device, ROV2PTOV (59N)must trip the component with some time delay. The setting must be above the highest1MRK 505 370-UUS A Section 8Voltage protectionBusbar protection REB670 2.2 ANSI 251Application manual
